This invention relates to electrodeless light sources driven by high frequency power sources and, more particularly, to the use of radioactive materials to aid in the starting of electrodeless light sources.
Electrodeless light sources which operate by coupling high frequency power to an arc discharge in an electrodeless lamp have been developed. These light sources typically include a high frequency power source connected to a termination fixture with an inner conductor and an outer conductor disposed around the inner conductor. The electrodeless lamp is positioned at the end of the inner conductor. High frequency power is coupled to a light emitting electromagnetic discharge in the electrodeless lamp. A portion of the termination fixture passes radiation at visible light frequencies, thus permitting use of the apparatus as a light source.
The electrodeless lamp in its operating condition represents a relatively low impedance of approximately a few hundred ohms. However, in the off state the impedance of the lamp is high. Since the termination fixture is designed to effect an impedance match to the operating impedance of the lamp, thus obtaining maximum transfer of power from the source to the arc discharge, there exists in the off state a mismatch between the lamp and the high frequency power source. This off-state mismatch creates a problem in starting a discharge when power is first applied to the light source. In the mismatched condition, the electric field in the lamp may be insufficient to cause starting. A tuning element located in the termination fixture is used for starting in U.S. Pat. No. 4,002,944 issued Jan. 11, 1977 to McNeill et al. A resonant condition is created which causes a strong electric field to initiate breakdown and excitation of the fill material within the lamp.
The use of ultraviolet light sources to start the discharge in electrodeless lamps is described in U.S. Pat. No. 3,997,816 issued Dec. 14, 1976 to Haugsjaa et al. An ultraviolet source illuminates the electrodeless lamp and, in combination with a high frequency electric field from the power source, induces starting of the electrodeless lamp. The function of the ultraviolet flux is to predispose loosely bound charges upon the inner surface of the lamp or free charges in the gas contained in the lamp envelope. The charges are then available to be acted upon by the applied high frequency field so that collisional ionization and breakdown ensue, thus initiating discharge. Either a glow lamp or a spark generating device is located in the space between the inner and outer conductors of the termination fixture. Ultraviolet light sources were also utilized in electrodeless light source starting systems in U.S. Pat. No. 4,041,352 issued Aug. 9, 1977 to McNeill et al. and in U.S. Pat. No. 4,053,814 issued Oct. 11, 1977 to Regan et al. R. J. Regan, "Electrodeless Light Source with Self-Contained Excitation Source", Ser. No. 952,765, filed Oct. 19, 1978 and assigned to the same assignee as the present invention, describes a self-contained ultraviolet starting aid for electrodeless light sources.
While ultraviolet starting aids give generally satisfactory results, they have certain disadvantages. The ultraviolet source is normally used in conjunction with circuitry which operates to remove power from the ultraviolet source after electrodeless lamp starting has occurred. Both the ultraviolet source and its associated circuitry add complexity to the light source and result in increased cost and lower reliability.
Ionizing nuclear radiations, derived from various isotopes of the elements, may be used to result in the same effects produced by ultraviolet radiation to assist starting of electrodeless light sources. The dominant radioactive emissions associated with the natural decay of the radioactive elements are beta particles, alpha particles, and gamma rays. Each type of radioactive emission has unique properties which determine how it can be used in the present invention.
gamma rays are energetic photons, as is well known, and are the most penetrating of the three emanations. Such rays typically possess energy in the 0.04 to 1 MEV range and penetrate materials such as aluminum from approximately 1 to 10 cm, respectively. Energy loss occurs largely by photoelectric effect as gamma rays are absorbed in matter so that ionized and excited atoms and molecules are left in the absorption path.
Beta particles are energetic electrons which possess, typically, energy in a range similar to that cited for gamma rays. However, beta particles are much less penetrating, ranging from 0.001 cm to 0.1 cm in aluminum for energies between 0.04 to 1 MEV, respectively. Their adsorption in matter results in ionized and excited atoms and molecules by collisional processes.
Alpha particles are helium nuclei which are typically emitted with energies in the several MEV range. Their range in matter is much less than that of either gamma rays or beta particles. In aluminum, for example, alpha particles penetrate less than 0.001 cm and only a few centimeters in air at standard conditions. Penetration of lamp envelope material will be similar to that cited for aluminum in each case above.
The prior art contains examples of the use of radioactive materials in gaseous discharge devices for the purpose of rapid initiation of breakdown in the gas. In all known prior art, it is the purpose of the initiating aid to yield a very short time interval between application of the driving field and breakdown in the gas.
A pulsed electrodeless illuminator is described in U.S. Pat. No. 3,648,100 issued Mar. 7, 1972 to Goldie et al. and methods are taught for achieving rapid turn-on and turn-off characteristics. A source of beta radiations, such as 0.1 microcuries of cobalt 60 or 0.3 microcuries of heavy hydrogen, internal to the lamp envelope is utilized. It is stated that the electric field acts directly on the beta particles and increases their energy until ionization of the gas occurs. use of radioactive emissions other than beta particles is not described. A beta particle emitter was used to promote rapid breakdown of the gas in a discharge device in U.S. Pat. No. 3,705,319 issued Dec. 5, 1972 to Goldie et al. Tritium, a beta emitter, was absorbed in titanium or yttrium and was separated from the discharge volume by a thin deposit of silicon dioxide.
In addition to the above-described patents, the following United States Patents, which may be of interest, relate to electrodeless lamps, usually at least one of the patentees of each patent is an applicant of this application, and all patents have been assigned to a common assignee.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 3,942,058 Haugsjaa et al. March 2, 1976 3,942,068 Haugsjaa et al. March 2, 1976 3,943,401 Haugsjaa et al. March 9, 1976 3,943,402 Haugsjaa et al. March 9, 1976 3,943,403 Haugsjaa et al. March 9, 1976 3,943,404 McNeill et al. March 9, 1976 3,993,927 Haugsjaa et al. November 23, 1976 3,995,195 Haugsjaa et al. November 30, 1976 4,001,631 McNeill et al. January 4, 1977 4,001,632 Haugsjaa et al. January 4, 1977 4,002,943 Regan et al. January 11, 1977 4,065,701 Haugsjaa et al. December 27, 1977 4,070,603 Regan et al. January 24, 1978 ______________________________________
Also of interest is the following United States Patent which relates to electrodeless lamps. 3,787,705 Bolin et al. Jan. 22, 1974